The proposed research is designed to examine molecular, morphological, and physiological events which accompany phenotypic expression in primary monolayer cultures of myogenic cells isolated from the chicken embryo. Organization of the the chicken genome is being examined by isolation and characterization of satellite DNA's and by analysis of reassociation kinetics of DNA fragments of various known lengths. Molecular hybridization studies are proposed to examine types of RNA synthesized at various stages of muscle differentiation and in BUDR-inhibited myoblasts. Studies are also proposed on the physiological maturation of muscle fibers developing in culture, utilizing fluorescent probes and measurements of the effects of various pharmacological agents on contractile activity.